1. Field of the Invention
The present invention relates to a color measurement device which measures the color of a patch for color measurement formed on a color measurement object, and an image forming apparatus, such as copying machines and printers of an ink-jet type and an electrophotographic type, provided with the color measurement device.
2. Description of the Related Art
In recent years, color image forming apparatuses such as a color printer and a color copying machine are required to enhance an image quality of an output image. Particularly, the stability of an image gradation or an image color gives large influence on a quality of an image. However, in the color printer, a color tone of an obtained image results in changing the color tone due to a change of an environment such as temperature and humidity and service in a long period. Accordingly, in order to realize a stable color tone, it is necessary to detect the color tone of the image by using a color measurement device, and to feed back the color measurement result to an image formation condition of the image forming apparatus.
Conventionally, the following types of color measurement devices are known as the color measurement device which measures the color tone of a color measurement object such as printed matter. First, a filter type (tristimulus-value direct-reading type) color measurement device is known which irradiates the color measurement object with white light, receives the reflected light with a photo-sensor through color filters of RGB, and thereby measures the intensity of every color component. In addition, a spectroscopic color measurement device is known which disperses the wavelength of the reflected light by using a diffraction grating, a prism or the like, then detects the intensity of every wavelength with a line sensor, and determines the spectral reflectivity of the color measurement object by performing a calculation in consideration of the wavelength distribution of the detected dispersed light, the wavelength distribution of light of a light source, a spectral sensitivity of a sensor and the like.
FIG. 16 illustrates a structure of a spectral color measurement device.
A spectral color measurement device 100 has a line sensor 101 which detects the dispersed light which has been dispersed. A light source 102 is formed of a white LED, a halogen lamp, an LED of three colors of RGB or the like. The light source 102 has the luminescence wavelength distribution which covers the whole visible light.
White light 105 which has been emitted from the light source 102 is incident on a color measurement object 104 at an illuminating angle of approximately 45 degrees, and turns into scattered light according to optical absorption properties of the color measurement object. A part of scattered light 106 is taken in a lens 107 to turn into parallel light, and then is incident on a diffraction grating 108 at an incidence angle of 0 degree, thereby being dispersed. The dispersed light which has been dispersed is incident on the line sensor 101.
Lights having different wavelength ranges are incident on pixels of the line sensor 101, respectively, and the intensity of every wavelength of a dispersed light which has been reflected by the color measurement object 104 is obtained from an output obtained from each pixel.
In such a spectral color measurement device, a tungsten lamp or the like which has an output in a wide wavelength zone has conventionally been used as a light source. However, a fine distortion of an optical system, which is produced by heat emanated from a light source, results in lowering the color measurement precision, and there are requirements for the color measurement device to be miniaturized and the like. Then, in recent years, a white LED which is small, can provide high luminance and generates a small amount of heat has been desired to be used as a light source. A conventionally well-known white LED includes a blue light-emitting chip, for instance, formed of an InGaN system, and a yellow fluorescent member, and emits light having a luminescence spectrum as is illustrated in FIG. 9. Light to be emitted from the blue light-emitting chip to be used here has a central wavelength of generally approximately 450 nm, and fluorescent light to be emitted from the yellow fluorescent member used here has a central wavelength of approximately 570 nm.
However, on the other hand, when measuring the color of the color measurement object such as printed matter with these color measurement devices, it is desired to measure the intensity in a wavelength zone that is equal to or more than 380 nm, or equal to or less than 780 nm in strict color measurement or in a wavelength zone of 400 nm to 700 nm even in the case of simple color measurement, as is described in JIS 28722, for instance. When the general white LED described previously is used as a light source, such a problem occurs that S/N properties are lowered and exact color measurement cannot be performed, because outputs in the vicinity of 400 nm and in the vicinity of 700 nm, and an output in a region (portion A in FIG. 9) between a luminescence center and a fluorescence center are low.
With addressing problems, in Japanese Patent Application Laid-Open No. 2007-093273, a precision for a signal in the wavelength region in which the output is low is compensated by using a calculation technique such as interpolation and extrapolation based on reflection characteristics which have been determined separately. In addition, in Japanese Patent Application Laid-Open No. 2003-014545 and Japanese Patent Application Laid-Open No. 2008-298579, an output from the above-described wavelength zone is obtained by using a plurality of LED packages.
However, the calculation technique which calculates reflection characteristics and is proposed in Japanese Patent Application Laid-Open No. 2007-093273 is different from a method of calculating original signal components, and still has a difficulty in a viewpoint of the accuracy of color measurement. In addition, even though the color tone of the image is detected in such a state that a color measurement precision thus cannot be secured, and the color tone is fed back to a process condition of the image forming apparatus, the stability of the color tone cannot be expected.
On the other hand, the method proposed in Japanese Patent Application Laid-Open No. 2003-014545, or Japanese Patent Application Laid-Open No. 2008-298579 results in causing such a problem again that the color measurement precision results in being lowered due to the increase of calorific value or the size of the color measurement device becomes large, because a plurality of LEDs is used.